In the Laboratory
Reticulation Study and Molding of Unsaturated Polyester Composites Sophie Bistac Institut Universitaire de Technologie, Département Science et Génie des Matériaux 61, rue Albert Camus F. 68093 Mulhouse cedex France;
[email protected] This paper describes a laboratory experiment involving polyester composites, especially sheet molding compounds (SMC), to investigate important aspects of composite science (polymerization chemistry and the molding process). During the practical work, the reticulation reaction of SMC is studied by differential scanning calorimetry in order to optimize the temperature and time of the molding process. Composites plates are also molded by compression under a heating press. Composition of Sheet Molding Compounds Sheet molding compounds are often used for the exteriors of automotive body panels, doors, spoilers, etc. because of the weight saving, lack of corrosion, product-design freedom, high heat and chemical resistance, and high strength-to-weight ratio they offer (1–3). SMC are are filled glass reinforced thermosetting polyester sheet molding compounds that contain an unsaturated polyester resin blended with styrene monomer. The filler is generally calcium carbonate and the reinforcement consists of chopped E-glass fibers. In addition to these major ingredients, the mix contains a peroxide catalyst and other additives. SMC is prepared by premixing the liquid (polyester and styrene) and dry (fibers, fillers) components and forming the sheets on a machine. SMC products are generally molded by compression in a heating press. SMC sheets can be obtained from SMC manufacturers. The reticulation of SMC is followed by differential scanning calorimetry (DSC), which allows students to determine the conditions of the molding process. SMC samples are then molded by compression. Experimental Procedure
Reticulation Study by DSC SMC products contain an unsaturated polyester resin. Reticulation, and therefore curing, of the SMC results from the reaction (radical copolymerization) between the unsaturations of the polyester chains and the unsaturations of the styrene monomer. This exothermic reaction, which leads to a three-dimensional network of polyester chains linked by styrene bridges, can be studied by DSC to determine the temperature, enthalpy, and length of the reticulation. These results allow students to determine and optimize the temperature and time of the molding process. The glass transition of the polyester resin can also be studied before and after reticulation. DSC analysis was performed with a DSC 30 purchased from Mettler. SMC samples (15–30 mg) were analyzed, before molding, from 20 to 200 °C at a scan rate of 10 °C min{1. A typical DSC curve for SMC is presented in Figure 1. An exothermic peak, attributed to the copolymerization reaction, is observed. The polymerization begins at 110 °C, and the maximum of the reaction occurs at 134 °C. The polymerization
Figure 1. Typical DSC curve of SMC composite showing the exothermic peak during copolymerization. Sample size, 25.800 mg; scan rate, 10.0 °C min{1; area under peak (between vertical lines), 1060 mJ or 41.1 J per gram of sample; peak location on x-axis, 134.3 °C ; peak height, 11.7 mW.
time is close to 4 minutes. Appropriate conditions for the molding process are thus 140 °C and 4 minutes.
Molding of the SMC under Heating Press SMC plates (10 × 10 cm, 4 mm thick) can be obtained by compression molding. SMC sheets are cut, measured by weighing, and put into the mold. SMC plates are obtained under pressure (100 bar) at 140 °C for 4 min. Heating and pressure allow the resin to flow, filling the mold, and the peroxide decomposes; thus reticulation can occur by copolymerization of the polyester resin with the styrene monomer (4, 5). After curing, the sample is removed from the mold and cooled. A DSC study of the molded material allows one to check if the reticulation is complete. The absence of any residual exothermic peak in the molded sample indicates total polymerization during molding. This analysis, performed after molding, allows one to optimize the molding conditions. Conclusion SMC composite is a good material for laboratory study of polymerization chemistry and the molding process. Literature Cited 1. Lucas, J. C.; Borrajo, J.; Williams, R. J. J. Polymer 1993, 34, 1886–1890. 2. Watanabe, T.; Yasuda, M. Composites 1982, 13(1), 59–65. 3. Chaturvedi, S. K.; Sun, C. T.; Sierakowski, R. L. Polym. Compos. 1983, 4, 167–171. 4. Toorkey, R. F.; Rajanna, K. C.; Sai Prakash, P. K. J. Chem. Educ. 1996, 73, 372–373. 5. Huang, Y. J.; Leu, J. S. Polymer 1993, 34, 295–304.
JChemEd.chem.wisc.edu • Vol. 76 No. 11 November 1999 • Journal of Chemical Education
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